To make the piri-piri sauce, preheat the oven to 180C/350F/Gas 4. Place the chillies on a roasting tray and roast them for 10 minutes.
Cool and roughly chop the chillies. Place the chillies, garlic, salt, oregano, paprika, olive oil and vinegar in a saucepan, and simmer for 2-3 minutes.
Allow the mixture to cool, then blend it to a purée in a jug blender or food processor. Store in a lidded container at room temperature; it will keep for about a month. Shake before using.
Place the spatchcocked chicken in a sealable plastic bag. Add half the piri-piri sauce, spreading it evenly over the chicken. Seal and marinate in the refrigerator for at least one hour.
Preheat the oven to 200C/390F/Gas 6, and preheat a large griddle pan on the hob. Alternatively, light your barbecue.
Season the marinated chicken, and cook it on the griddle pan for 2-3 minutes on each side, until golden brown.
Transfer the griddled chicken to a roasting tray and roast in the oven for 30 minutes, until cooked through. Alternatively, place the chicken on a medium heat barbecue, covered, for 10-15 minutes on both sides or until cooked through, basting regularly with the remaining piri-piri sauce.
Serve with chips and salad.